Three well-known studies give an indication of the importance of epidemiology to this field. These include a study of homicides committed by mentally disordered offenders in Germany, a study of correlates of antisocial behavior in American children, and a 30-year longitudinal study examining factors associated with criminal behavior in a sample of London families. The Institute of Psychiatry in London has used epidemiology for research explorations into the prevalence of mental disorders including epilepsy among inmates, the relationship between violence and disease, and the outcome of patients deemed incompetent to plead. Nonetheless, if used unthinkingly, epidemiology can mislead as well as inform. One of these misconceptions is Penrose's Law, which presumably states that there is an inverse relationship between mental hospital and prison populations. In the UK, recent surveys have found that substance abuse and personality disorder are the most common psychiatric diagnoses among offenders. 6 tables and 40 references